Mechanically driven chronograph watches comprise generally a central seconds hand which upon actuation of the start mechanism is coupled to the gear train and advances step by step at the frequency which is determined by the balance wheel and hairspring. If this frequency is 18,000 alternations per hour, the seconds hand will progress step by step 18,000:3,600=5 steps per second, i.e. through an angle of 1.2.degree. per step. If the hand moves over a dial on the order of 30 mm of diameter, this stepwise displacement of its extremity over the hours division is easily visible (arc of about 0.3 mm). In this case the smallest period which can be read is on the order of a fifth of a second.
Attempts have already been made to obtain chronograph watches of the electronic type where the balance wheel and hairspring mechanism is replaced by a quartz time-base which controls by means of a frequency divider a stepping motor which is caused to step once per second.
This is the case for example in the construction described in the U.S. Pat. No. 3,884,035 which provides however at least two driving motors, one for the timekeeping display and the other for the chronograph function display. In a variant of this arrangement the frequency divider circuit includes two outputs at different frequencies, one being at 1 Hz in order to control the motor of the timekeeper and the other of at least 8 Hz so as to control via a starting switch and an auxiliary frequency divider, the motor or motors of the chronograph. When the switch is operated the auxiliary divider begins to count the 8 Hz pulses received from the principal frequency divider in order to deliver at its output one control pulse for every 8 impulses received at its input. In this manner it will be understood that the display error may attain the value of -1 second if the chronograph function is started just before the arrival of the eighth pulse. It will be noted in passing that this error could be .+-.1 second at most if there were no auxiliary frequency divider and the chronograph motor were controlled directly by the 1 Hz pulses from the principal frequency divider.
It remains nonetheless the case that for a chronograph function even an error of -1 second remains important relative to the reading of a fifth of a second which one may obtain with a classical mechanical chrongraph. To overcome this difficulty the U.S. Pat. No. 3,884,035 cited above foresaw as a variant the utilization of a further motor driven at a frequency higher than 1 Hz which then displayed fractions of a second. Initially, it will be noted that the complication of such a system is a serious obstacle to its commercial realization. Furthermore, from the admission of the inventor, the utilization of several motors can considerably increase the current consumption to a point such that a supplementary energy source may be necessary.
The watch chronograph described in the British patent document No. A-2,028,545 likewise employs two stepping motors one of which drives the wheel train for indicating the time of day and the other, the chronograph wheel train. The chronograph motor is controlled by pulses provided from the divider circuit at a frequency of 10 Hz. The central seconds hand which forms part of the chronograph display mechanism is driven at a speed of rotation ten times the normal speed so that it effects one sweep of the dial in 6 seconds. In this manner it is possible to read a tenth of a second. However, the reduction ratios which are necessary in the chronograph wheel train to drive a minutes counter and an hours counter are then such that encumbrance of the movement becomes important. It is likewise to be noted that the reading of periods of a duration less than 1 minute but greater than 6 seconds is not facilitated.
In mechanical chronograph watches it is current practice that the chronograph mechanism in its quasi totality is borne on an independent support which is removably fastened to the support of the movement as taught for example by Swiss Pat. Nos. 225 004 and 527 462. This manner of construction can be advantageous without requiring an increase in the mechanical power of the motor equipping the base movement, said motor possessing sufficient reserve to drive the overload presented by the chronograph mechanism.
This same principle has been applied more recently to a basic movement employing a quartz and a stepping motor. However, when it has come to the mounting of the chronograph mechanism on the basic movement employing a single stepping motor normally driving only a simple timekeeper, it has been noted that the energy furnished by the motor is at the limit of acceptability which is understandable since the motor must drive an additional load for which it has not been calculated. From this situation there results insecurity of operation appearing above all with the application of special external constraints. To overcome these difficulties consideration has been given to increase the power to be furnished by the stepping motor, this leading in turn to an increase in its dimensions and from there modifying in a substantial manner the thickness and the arrangement of the entire basic movement. One thus departs from the prime purpose taught by the Swiss Pat. Nos. 225,004 and 527,462 as cited which permitted the utilization of a standard universal caliber apt to be employed in a simple timekeeper of minimum thickness or in a chronograph watch.
It will be likewise noted that the known chronograph watch as cited in the preceding paragraph, is equipped with a stepping motor which steps at one step per second. As has been explained above, the measurement error may extend to .+-.1 second which in many cases is not acceptable.